The outer membranes of Gram-negative bacteria are barriers to diffusion of many important nutrients such as iron-siderophores into the periplasmic space. To circumvent these limitations, active transporters with very high affinities for their transport ligands are located in the outer membrane. However the outer membrane lacks conventional energy resources of sufficient ion gradients or access to ATP. Instead, energy for active transport across the outer membrane is transduced from the cytoplasmic membrane proton motive force by three integral cytoplasmic membrane proteins, TonB-ExbB-ExbD. ExbB/D harvest the proton motive force and transmit it to TonB, which directly contacts the outer membrane transporter. The exact mechanism of energy transduction to the outer membrane transporter is unknown. Our long- term goal is to understand the mechanism of TonB-dependent energy transduction between the cytoplasmic and outer membranes using Escherichia coli as the model system. Because of the role it plays in iron acquisition the TonB system is a virulence factor for many Gram-negative pathogens. Our long-term goal is to understand the mechanism of TonB-dependent energy transduction between the cytoplasmic and outer membranes of Escherichia coli. Past in vitro studies have characterized binding of TonB periplasmic domains (lacking the essential transmembrane domain) to purified transporters. To date, actual transport has not been demonstrated. We will identify the residue-specific energy-dependent interactions that occur between active full-length TonB and the outer membrane transporter FepA during transport in vivo. This knowledge will be used to evaluate in vitro interactions between FepA and active full-length TonB. We will also identify unknown proteins in the TonB system and characterize the role they play.